Enough that the total investment is proportionate to the real cost of acquiring and operating the business and sufficient to make it succeed. There is no fixed minimum; the measure is the ratio of what has been committed to what the business genuinely needs.
Substantial is a proportion, not a price tag
The E-2 substantiality test compares the amount invested with the total cost of purchasing an established enterprise or establishing a new one. For an existing mechanical contractor the total cost includes the asset price, licensing and insurance transfers, vehicle and equipment needs, and the operating reserve required to carry payroll and materials until receivables turn over. An investor who pays the full price but arrives with no reserve may be told the investment is insufficient to ensure the business will operate, while one whose committed funds cover the price and a realistic reserve has met the test comfortably.
The business plan should show the calculation openly: what it costs, what has been committed, and what remains. Separately, the enterprise must not be marginal, meaning it should do more than support the investor's household; an existing crew, contracts in hand and a plan to add staff answer that point. Government fees for the visa application and any petition are on the State Department and USCIS fee schedules and are small beside the working capital question.